In an FT-IR spectrometer, an interferometer of the Michelson type splits an input light beam into a reflected beam and a transmitted beam by means of a beam splitter. Each split beam travels along its own path to a return mirror which deflects it back to the beam splitter along the same path. One of the return mirrors is stationary whilst the other is movable, typically along a linear path between two limits equidistant from a datum position. At the beam splitter the return split beams recombine along a common output path leading to a photodetector via a sample station.
If the movable mirror is at its datum position the optical path of the two split beams is the same so that when those split beams return to the beam splitter, they constructively interfere. This results in a large signal being produced at the photodetector and this is known as the center burst.
If the movable mirror is shifted towards the incoming split beam, the optical path of that beam decreases and conversely if it is moved away, the optical path is increased. Thus, as the movable mirror is moved from one limit to another, two complete series of optical path difference values of opposite signs are generated and this travel is referred to as an OPD scan. The output signal of the photodetector during an OPD scan is a series of superimposed electrical sine waves of different frequencies and amplitudes. This signal is known as an interferogram.
Such interferometers include a reference light source, typically a laser, which is used to measure the optical path difference. The reference fringes created during an OPD scan are sensed by a photodetector which generates a reference fringe signal which is a sine wave.
When no sample is present at the sampled position, the detector signal is the emission interferogram of the light source, typically an infrared source. When a sample is present, the output signal of the detector is the interferogram of the sample superimposed upon that of the light source. By taking the Fourier transform of the source interferogram and the Fourier transform of the sample interferogram superimposed upon that of the source, it is possible to obtain the spectrum of the sample.